An upset win by Trevallyn has lumped both Riverside teams in a sudden-death semi-final next week in the Tennis North spring roster.
Trevallyn won't play finals but shaped the top four by beating Legana on games difference and pulling the West Tamar side down to fourth.
LRTC's victory against Riverside Blue was also decided on games. Top seed Daniel Warren beat Matthew Webb 6-2 before Riverside Blue's Josh Partridge squared the ledger with a 6-4 win over Alex Suter.
ELSEWHERE IN SPORT
LRTC's Jordan Lovick won the remaining singles against Campbell Young and despite losing two out of three doubles matches, LRTC tallied 31 games to 24 to take the night's honours.
Andrew Corbett gave Legana a flying start with a 6-0 win against Trevallyn's Glenn Clifford but Legana's Andrew Faulkner fell 6-1 to David Beattie.
Trevallyn captain Phil Bonney beat Legana's Riley Dunn before the doubles matches decided the rubber, 3-27 to 3-22.
Riverside White's Oliver Hadley and Manning Mitchell had solid singles wins against Newstead Tennis & Squash Centre's Kiran Naidoo and Sam O'Shea respectively in a solid tune-up for the semi-finals.